In "An Approach to Minimum Power Consumption in Low Temperature Gas Separation", Trans. Instn. Chem. Engrs., Vol. 36, 1958, G. G. Haselden identifies irreversibility of distillation columns as a key source of inefficiency in the operation of cryogenic air separation processes. He points out that, because of the change of slope of the reboil requirement curve in the lower part of an air separation column occuring at a vapor composition of about 50% oxygen, it is possible to make a simple approach towards ideal column operation by adding about half the reboil heat at a single level in the column a little below the feed, say at a temperature of 88 K, the remaining half being added at the terminal temperature of 92.7 K. He further observes that any practical attempt to approach ideal non-adiabatic column operation by the use of distributed heating and cooling sources operating over extended zones of the column will be most effective for moderate product purities. A cycle is proposed utilizing the column operating principles identified in the aforesaid paper. Even with the use of an auxiliary column, forty percent of the oxygen is produced at medium purity.
In U.S. Pat. No. 4,025,398, (G. G. Haselden) it is proposed that two distilling systems be arranged to interchange heat with each other in order to achieve a close approach to the kind of thermodynamic ideal discussed above. One such distilling system comprises a first column having a rectifying section in which there are varying amounts of reflux, and a second column having a stripping section in which there are varying amounts of reboil. Thermal linkage between the two columns is provided by taking vapor from the variable reflux column, pertially condensing it in the stripping column, and returning the resulting liquid-vapor mixture to the variable reflux column. The partial condensation takes place in passages formed in distillation trays of the stripping column. Heat is thus extracted from the stripping column and is transferred to the variable reflux column. In the drawings accompanying the aforesaid U.S. patent specification, four trays are shown provided with such heat exchange passages and hence there are four associated liquid outlets from the variable reflux column and four associated inlets to the variable reflux column for the liquid-vapor mixture that is formed by partial evaporation of the liquid in the heat exchange passages.
The streams of vapor are taken from the variable reflux column just below the level of the chosen trays and the liquid-vapor mixture is returned to the column just above the respective trays. Although the proposals in U.S. Pat. No. 4,025,398 represent an advance in the art, it is difficult to fabricate a distillation system as described therein which will operate at cryogenic temperatures. First, it is not easy to provide a piece of apparatus that can function adequately as both a distillation tray and as a heat exchanger to enable the partial condensation of the vapor from the variable reflux rectifier to be effected. Moreover, in a practical distillation system operating at cryogenic temperatures a large number of trays are typically required. In order to approach the thermodynamic ideal set out in U.S. Pat. No. 4,025,398 with such a system, it becomes necessary to provide a multiplicity of passages extending from the variable reflux rectifier to a large number of heat exchangers in the stripping column and a further multiplicity of passages for returning the resulting liquid-vapor mixture to the variable reflux column.
The use of the process described is U.S. Pat. No. 4,025,398 to produce oxygen is discussed in "Energy Conservation and Medium Purity Oxygen", J. R. Flower, I. Chem. E. Symposum Series No 79, pp F5-F14. The process is summarized as involving the taking of a number of vapor sidestreams from a first column and condensing them in heat transfer baffle elements immersed in the two phase mixtures on selected distillation stages of the second column. The condenser products would pass back to stages in the first column where the compositions matched. From analysis of this cycle, it was found that the advantages of distribution of heat flux decreased sharply as the product (oxygen) purity changed from 95 to 99% and that the critical part of the design involved the matches at the base of the second column for liquid (oxygen) compositions greater than 85%. It is therefore concluded that the cycle is primarily useful for producing medium purity oxygen. It is further reported that, where there are no suitable heat transfer baffles, papers have suggested utilizing a series of reboiler - condensers situated between the first and second columns, each fed by a separate vapor sidestream and a separate liquid sidestream. The condenser products and evaporator products are returned to the first and second columns. It is reported that the advantages of using such existing heat exchange equipment are offset by a requirement for higher air feed pressures partly as a result of liquid hydrostatic effects.
It can therefore be seen that these existing proposals for distributing the necessary heat and refrigeration over a distillation column generally require a multiplicity of links between a pair of columns, and in the example of the production of oxygen are not effective to produce high purity oxygen. In general, the industrial demand for high purity oxygen is far greater than that for what is termed medium purity oxygen. Moreover, when medium purity oxygen is produced, it is generally not possible to obtain in the distillation system a sufficient local concentration of argon to justify the inclusion of an additional column to produce pure argon.
Our analysis of the distillation of air shows that disproportionately more energy is required in producing a given percentage change in a composition containing less than 80% nitrogen than in one containing more than 80% nitrogen. Accordingly, in air separation there is a greater need for reboiling compositions between air and pure oxygen than there is for condensation of compositions between air and pure nitrogen. This appreciation of the relative merits of `intermediate` reboil and `intermediate` condensation is not shown in the prior art. Indeed, we have noted two prior proposals, U.S. Pat. No. 2,812,645, and German Patent No. OLS 2 202 206, which disclose an intermediate condensation step but not an intermediate reboiling step.
Therefore, in accordance with the present invention, there are provided a process and apparatus for separating air in which reboil is provided at more than one level in a distillation column employed to separate the air, while making possible the production of an argon product and a relatively pure oxygen product.